Do some research on the candlepower of the bulbs - you'll generally need the brightest LED you can buy in order to get the equivalent brightness of the original incandescent bulb.

If you decide to install colored LEDs, you will lose any color in your gauges other than the color of the LEDs. A blue LED only creates light in the blue spectrum, so if your redline is designed with a red filter to allow it to show as red, you'll no longer see it because the LED doesn't create any light in the red spectrum to show through the filter. The crappy cell phone pic of my Miata's cluster below shows what I'm talking about.



The OEM gauge markings have a slight green filter on them (as you can see on the HVAC controls) and the aftermarket gauges have no filter, which makes them far brighter. The redline markings on the tach are virtually invisible now. Since I took this photo I replaced the brighter LEDs in the aftermarket gauges with some slightly lower power LEDs to balance the light levels a little better, and installed LEDs in the HVAC controls also.

These are the two different LED types I ended up using - ordered them on Ebay from a seller in Hong Kong, which took a few weeks to get here but was half the cost of SuperbrightLEDs.com.